Which of the following best summarizes why the missouri compromise needed to be replaced by the compromise of 1850?

a. members of congress wanted to end the question of slavery in the u. s.
b. the u. s. wanted to sell the territory gained after the mexican-american war to build railroads, and the missouri compromise prevented it.
c. congress decided that the people should be able to decide whether the territory gained in the mexican-american war should be allowed to have slavery.
d. the territory gained after the mexican-american war was mainly below the missouri border, and u. s. leaders believed slavery would be necessary to ensure settlement.